<!DOCTYPE html>
<style>

.a { }
#f {
  position: absolute;
  left: 120px;
  top: 120px;
  width: 250px;
  height: 250px;
  border: 0;
  outline: 100px solid #fc8;
}

</style>
<div class="a"></div>
<iframe id="f" srcdoc="
  <style>
    body { will-change: transform; }
    #b { height: 200px; background: blue; opacity: 0.2; }
  </style>
  <div id=b></div>"></iframe>
